XPost: sci.physics, sci.space.tech   
   From: root@mauve.demon.co.uk   
      
   In sci.space.tech John Schoenfeld  wrote:   
   > Timo Nieminen  wrote in message news:<   
   ine.LNX.4.50.0307110958020.29738-100000@kolmogorov.physics.uq.edu.au>...   
   >> On Thu, 10 Jul 2003, George Kinley wrote:   
   >>   
   >> > if there is no Atmosphere, where do rockets that go in Space get thrust   
   from   
   >>   
   >> Stand on a skateboard on a smooth flat surface, with a bag of rocks. Throw   
   >> the rocks in one direction, and you'll go in the other direction (except   
   >> for the inconvenient effects of friction - use bigger rocks and you should   
   >> see an effect). The rocket works exactly the same way. Throw mass away in   
   >> one direction, and you go in the other direction. "Conservation of   
   >> momentum" is what it's all about.   
   >   
   > The direction of the rockets acceleration is irrespective of the   
   > direction of mass explusion. I could build a rocket that expels its   
      
   Err, no it's not.   
   > mass upwards and it would still accelerate upwards. It is the   
   > explosion in the combustion chamber that pushes out in ALL directions.   
   > The upwards push makes contact with the top of the combustion chamber   
   > transferring a net upwards momentum to the rocket. However, the   
   > downward push from the explosion does not make contact with any   
   > structure from the rocket and escapes - so the net momentum is   
      
   In some ways, this is a not utterly inaccurate description of the   
   rocket combustion chamber, up to the throat.   
   However, at the throat the rules change.   
   The gas enters the widening nozzle, and rapidly expands.   
   The expanding gas causes a pressure against the nozzle walls, and   
   the component of the force which acts opposite to the direction of   
   the exhaust is transferred through the nozzle walls.   
   If you just have a hole in the combustion chamber, the exhaust   
   pretty much spreads over a hemisphere, and you get relatively   
   little thrust.   
   A deLaval (sp?) nozzle, produces a much higher thrust, as the   
   exhaust is not a hemispherical expanding gas, but a relatively   
   cold jet going in the opposite direction.   
      
   > upwards. It should be noted that the actual upwards acceleration has   
   > nothing to do with the output direction of the expelled mass (I could   
   > very well route the mass from the explosion pushing downards via the   
   > top or side, although this is both extremely difficult and very   
   > inefficient).   
      
   This is totally incorrect.   
   A few seconds with a balloon, a bit of tape to tape the neck down to the   
   balloon should convince you of this.   
      
   >   
   > Also note that you don't even need mass to escape from a rocket.   
   > Inertial propulsion is not prohibited in physics if you think about   
   > it. All that needs to be done is to make the upwards push a greater   
   > impulse than the downwards push - the rocket would essential jerk its   
   > way upwards - after all there is no "conservation of displacement"   
   > with such an inertial system.   
      
      
   This is also totally incorrect.   
   Conservation of momentum means that you have to expel something.   
      
   --   
   http://inquisitor.i.am/    |  mailto:inquisitor@i.am |             Ian   
   Stirling.   
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